Falchion of Law

Weapon (longsword), legendary (requires attunement by a lawful character)

You gain a +2 bonus on attack and damage rolls when you use this weapon. If you hit an aberration or demon, you do an extra 3d6 damage. When you attack a creature that has at least one head with this weapon and roll a 20 on the attack roll, you cut off one of the creature’s heads. The creature dies if it can’t survive without the lost head. A creature is immune to this effect if it is immune to slashing damage, doesn’t have or need a head, has legendary actions, or the GM decides that the creature is too big for its head to be cut off with this weapon. Such a creature instead takes an extra 6d8 slashing damage from the hit.